Topic 6.2 - Aromatic Compounds and Amines Flashcards
(29 cards)
What structure does benzene have?
What is its molecular formula?
- Benzene has a planar cyclic structure
- C6H6
Why does benzene have a ring of delocalised electrons?
Each carbon atom only has three single covalent bonds when normally they are supposed to have four.
Fourth covalent bond (electrons in p-orbitals) for a ring of 6 delocalised electrons above and below plane of ring
What three pieces of evidence suggest that benzene has a delocalised structure?
1) Does not undergo addition reactions
- Does not decolourise in bromine water
2) C-C bonds are the same length
- C=C bonds would be shorter
- High electrons density so Cs closer together
3) Lower enthalpy of hydrogenation
- 152 KJmol-1 less exothermic than expected
- More beefy needed to break bonds in benzene
- Delocalised electrons arranged in ring
- Electron density shared over more atoms
What does benzene attract and why?
Benzene is a region of high electrons arranged density so attracts electrophiles (electron pair acceptors)
What are the two reactions that take place in electrophilic substitution?
- Friedel - Crafts acylation
- Nitration mechanism
What is the formula to form NO2+ ?
What are the conditions?
2H2SO4 + HNO3 > NO2+ + H30+ + 2HSO4-
NO2+ = nitronium ion (electrophile)
Conditions: warm concentrated nitric acid and concentrated sulphuric acid
What is the equation for H2SO4 catalyst reforming?
H+ + HSO4- > H2SO4
Why do nitration reactions have to take place in conditions below 55•C ?
Temperature above 55•C will cause multiple substitutions of NO2+
What are nitrocompounds used as?
Explosives as they decompose silently when heated
What are the conditions and products produced from Friedel - Crafts acylation?
- Benzene rings are used in many dyes and pharmaceuticals. However, due to their stability, they are fairly unreactive and difficult to make
- Products: phenylketone + HCl
- Conditions: Heat under reflux in a non-aqueous solvent
How does AlCl3 become a strong electrophile?
- Accepts a lone pair of electrons from the acyl chloride
- Increase polarisation of acyl chloride
- Carbocation formed
- Becomes much stronger electrophile that can react with benzene
What is the reaction between AlCl3 and acyl chloride?
RCOCl + AlCl3 > RCO+ + AlCl4-
What is the reaction between AlCl3 and acid anhydride?
RCOOCOR + AlCl3 > RCO+ + Cl3AlOCOR -
What are aliphatic amines?
What are aromatic amines?
Aliphatic amines = amines without benzene rings
Aromatic amines = amines with benzene ring directly attached to N
How do you know if an alien is produced during a reaction?
Fishy smell
What is the order of amines with increasing base strength?
- Aromatic amine
- Ammonia
- Primary amine
- Secondary amine
- Tertiary amine
Why are aromatic amines weak bases?
- Benzene ring draws electrons towards itself
- The lone pair on N becomes partially delocalised into benzene ring
- Electron density on N decreases
- Lone pair on N becomes less available
Why is ammonia a somewhat weak base?
- No benzene ring to pull lone pair of electrons away from N
- No alkyl groups to push electrons towards N
Why are aliphatic amines strong bases?
- Alkyl groups push electrons towards N
- Electron density on N increases
- Lone pair on N is more available
- Strong positive inductive effect
How do Quaternary ammonium salts work?
- Non-polar tail binds to grease and oils
- Polar head dissolves in water
- Cation helps cancel out negative charge
> hence acts as an anti-static agent in fabric softeners and conditioners
What are four uses for quaternary ammonium salts
- Cationic surfactants
- Fabric softeners
- Hair conditioners
- Detergents
Define a surfactant
- A compound which is partly soluble and partly insoluble in water
- At least one long hydrocarbon chain
Define cationic surfactant
- A surfactant which is positively charged
- Forms induced dipole-dipole interactions with non-polar substances and hydrogen bonds with water
Why do amiss act differently to amines?
Carbonyl groups pull electrons away from the NH2 group
